Quiet nacelle system and hush kit

ABSTRACT

A quiet nacelle system for jet aircraft engines includes an elongated nose cowl and nose dome of the nacelle and a sound-attenuating liner applied to the nose cowl, nose dome and fan duct components. The system is particularly suitable for JT3D-3B and JT3D-7 engines for use with DC-8 aircraft. The components may be combined as a hush kit for retrofit in existing nacelle systems.

BACKGROUND OF THE INVENTION

Prior to 1969, the aircraft industry paid little concern to commercialjet aircraft engine noise. In December 1969, the U.S. Federal AviationAdministration ("FAA") promulgated specific noise level regulations foraircraft under authority of Public Law 90-411. Existing airplanes wererequired to be certificated for compliance with Federal Air RegulationNo. 36 ("FAR 36"). Similar noise standards were prescribed byinternational civil aviation organizations (for example, "ICAO Annex16"). Several states and municipalities also established airport noiselevels. Thus, it became imperative for airframe and engine manufacturersand owners to take noise considerations into account in designing,building and using jet aircraft.

By December 1970, Douglas Aircraft Co., on behalf of the NationalAeronautics and Space Administration, had undertaken studies concerningfan-compressor noise from the Pratt & Whitney JT3D-3B engines used withDC-8-50/61 airplanes made by Douglas. The studies showed that noiselevels might be reduced in a short duct nacelle design having revisedfan inlet and exhaust ducts containing acoustically absorptive linings.Various configurations were tested. Although it was shown that typicallyup to 10.5 EPNdB ("effective perceived noise") reduction in the noiselevel could be achieved (at maximum certified landing weight and at apoint on the ground beneath a 3° landing-approach path one nautical milefrom the runway threshold), static take-off-rated gross thrust wasreduced by 2.5% and fuel consumption was increased by 3%. Moreover,direct operating costs were estimated to increase 4.4% and return oninvestment would decline about 6%.

At the same time, The Boeing Co. was conducting similar studies of noisereduction of JT3D engines used with Boeing 707-320C airplanes. Thestudies showed that noise reductions up to 15 EPNdB could be achieved onlanding approach. It was planned to accomplish this reduced noise levelby installing two acoustically treated rings in the engine inlet and byacoustically treating an extended 3/4 length fan duct configuration.That modification resulted in a range reduction of 200 nautical milesand a direct operating cost increase of more than 9%. Further, the useof rings may interfere with de-icing of the engines.

Thereafter, the major airframe manufacturers undertook substantialresearch effort toward developing retrofit kits for existing airplanesto attempt to meet FAR 36 noise level requirements. However, they wereunable to develop kits which would meet the noise requirements without,at the same time, degrading performance, increasing fuel consumption andunreasonably increasing costs. As a result of the unavailability ofretrofit hush kits, the value of existing airplanes fell significantlyas their useful lives neared an end. As of Oct. 1, 1979, there wereabout 155 DC-8 airplanes with JT3D engines in service by U.S. airlinesand about 218 in service by foreign airlines. Although the effectivedate of the FAR 36 noise requirements was extended on several occasions,and the requirements were modified, the regulation finally becameeffective as of Jan. 1, 1985. On that date, the existing DC-8 airplanesbecame essentially obsolete for use in the United States.

The studies which had been conducted showed that noise radiates from alow by-pass, fan jet engine in several directions. High-frequency fannoise radiates both forward through the air inlet cowl and aftwardthrough the exhaust ducts. Low-frequency jet noise generally radiatesrearwardly. At low engine thrust, the high-pitch whine of the fan ismore pronounced. At high engine thrust, the low-pitch jet rumble is morenoticeable.

Each noise component must be dealt with separately, as well as incombination. Generally, some studies showed that noise attenuationmaterial was useful for reducing some noise components. One type ofmaterial frequently used consists of honeycomb core cells bonded to aporous sheet on the airflow surface and an impervious sheet on therearward surface. Such noise attenuation material has been used inseveral different nacelle configurations for different airplanes.

Other studies showed that nacelle modifications could suppress enginenoise. However, these modifications which, it appeared, could succeed insuppressing noise would also severely degrade airplane performance orsubstantially increase fuel consumption.

Thus, although existing DC-8 airplanes could not be used, at least inthe United States, after implementation of the FAR 36 noise levelrequirements, no one had developed and certified with the FAA a systemto reduce the noise levels and thereby make the obsolete DC-8 airplaneseconomically viable.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an aircraft jet enginenacelle adapted to permit engine operation at low noise levels attake-off and landing.

Another object of the invention is to provide a nacelle system for a jetengine which reduces engine noise to a quiet level conforming withgovernmental airplane noise regulations, while maintaining airworthinessand flight performance of the airplane.

Still another object of the invention is to provide a retrofit kit forexisting jet airplanes which will permit continued operation ofairplanes which otherwise would not comply with governmental noiseregulations and could not be flown.

The present invention contemplates achieving these objects through useof a combination of nacelle structural modifications and properlypositioned sound attenuating material. To that end, the nose cowl of thenacelle is elongated and an acoustic liner is used on the airflowsurface. The nose dome is extended to conform to the nose cowlelongation and the nose dome extension has an acoustic liner on theairflow surface. The fan ducts are provided with acoustic liners on theairflow surfaces, and the splitters also are provided with acousticliners on the airflow surfaces. The preferred acoustic liner is asingle-layer honeycomb sandwich material.

The nacelle system of the present invention is particularly adapted foruse with Pratt & Whitney JT3D-3B and JT3D-7 engines, for use withDouglas DC-8-62 and DC-8-63 aircraft, but is also useful with similaraircraft and engines.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a DC-8 aircraft equipped with quietnacelles in accordance with the present invention.

FIG. 2 is a graph depiction of the reduced noise level contours achievedby the present invention as compared to an unmodified nacelle system.

FIG. 3 is a perspective view, partially cut-away, of the nacelle systemfor a DC-8 aircraft.

FIG. 4 is an elevational, cross-sectional view of the inlet cowl of thenacelle system.

FIG. 5 is an elevational, cross-sectional view partially cut-away, ofthe nose dome.

FIG. 6 is a perspective view, partially cut-away, of thesound-attenuating material.

FIG. 7 is a perspective view of the constant section fan duct assembly.

FIG. 8 is an exploded, perspective view of the constant section fan ductassembly.

FIG. 9 is a perspective view of the transition section fan ductassembly.

FIG. 10 is an exploded, perspective view of the transition section fanduct assembly.

DETAILED DESCRIPTION OF THE INVENTION

Generally, as shown in FIG. 1 of the drawings, the present inventionrelates to a jet airplane 10 having a plurality of nacelles 12 suspendedfrom the airplane wings 14 by pylons 16. Each nacelle 12 contains a jetengine adapted to propel the plane. The nacelles 12 are modified inaccordance with the present invention so as to operate at noise levelswhich meet governmental noise regulations, including particularly FAR36, and still maintain airworthiness and structural integrity underother parts of FAA regulations.

The nacelles 12 are shown in greater detail in FIG. 3. Each nacelle 12includes a housing comprising a nose cowl 20, a forward cowl 22 trailingthe nose cowl 20, and an aft cowl 24 trailing the forward cowl 22. Thenose cowl 20 defines the inlet 26 to the nacelle. The forward cowl 22and aft cowl 24 circumscribe the fan, combustion and turbine sections ofthe engine 28. The exhaust 30 of the engine is defined by the thrustreverser 32 and nozzle 34. Other than as described herein, each of thoseelements is a standard component in existing nacelle systems known tothose skilled in the art.

The nose cowl 20 of the nacelle 12 is modified in accordance with thepresent invention to have an extension or elongation beyond the nosecowl length conventionally employed in existing nacelle systems for DC-8airplanes in use at the present time. It has been found that anextension of approximately 12 inches provides, in combination with theother components of the system, particularly effective reduction innoise levels. A longer extension is unnecessary and adds weight to thesystem without improving performance. The term "elongated nose cowl", asused herein, means a nose cowl which has been extended to about thatoptimum length.

As best seen in FIG. 4, the nose cowl 20 comprises an outer barrel 36,which forms the exterior surface, an inner barrel 38 which forms theinterior airflow surface and a lip assembly 40 connecting the inner andouter barrels and which includes the anti-icing system for the engine.The inner barrel 38 is lined or bonded with sound-attenuating material42 on the airflow surface. However, the sound-attenuating material 42 isnot applied to the lip assembly 40, so as to avoid interference with theanti-icing system. It has been found that the forward edge of thesound-attenuating material 42 should be located about 6 inches behindthe lip assembly for that reason. Preferably, the material 42 forms aliner having two different thicknesses. It has been found that thematerial desirably has a thickness of about 3/4 inch for most of thelength of the inner barrel 38, but that the thickness should be reducedto less than 1/2 inch for the rearwardmost portion of the materialadjacent the engine attachment flange at the aft bulkhead 44. Thisfacilitates attachment of the engine 28.

The nose dome 50 extends from the fan 52 of the engine 28, as shown inFIG. 3. The nose dome 50 is bullet shaped and conforms to the shape ofthe inner barrel 38 of the nose cowl 20, so as to provide smooth airflowthrough the inlet 26. The nose dome 50 of the nacelle 12 is modified inaccordance with the present invention to have an extension or elongation54 adjacent the fan 52. The extension 54 preferably should be about 12inches in length to cooperate and provide effective sound attenuation incombination with the extended nose cowl 20. The pitotstatic PT 2 probeand anti-icing system incorporated in the nose dome are not affected bythe extension.

As best seen in FIG. 5, the existing nose dome 50 is not structurallychanged, but the extension 54 is attached between the rearward portionof the nose dome 50 and the fan 52. The entire extension 54 is lined orbonded with sound attenuating material 56 on the airflow surface.However, the sound-attenuating material 56 is not applied to theunmodified portion of the nose dome 52, so as to avoid changing theairflow characteristics in the inlet. It is desirable to avoid changingthe airflow liner and aerodynamic loft lines, because any such changesmay affect the performance and airworthiness of the plane.

Fan ducts 60 extend rearwardly from the engine fan 52. Compressed air isforced through the fan ducts 60 to provide increased thrust for theengine. The fan ducts 60 must be configured to fit in the narrow spacebetween the outer housing provided by the forward cowl 22 and aft cowl24 and the jet engine 28. A nacelle which has fan ducts 60 extendingfrom the fan 52 to the thrust reverser 32 is referred to herein as a"long fan duct" nacelle.

As shown in FIG. 3, the fan ducts 60 comprise pairs of bifurcation fanducts 62, constant section fan ducts 64 trailing the bifurcation fanducts 62, transition section fan ducts 66 trailing the constant sectionfan ducts 64 and aft section fan ducts 68 trailing the transitionsection fan ducts 66 and leading to the thrust reverser 32. It has beenfound that, in the combination of the present invention, nosound-attenuating material is necessary for the bifurcation fan ducts 62and aft section fan ducts 68.

The constant section fan duct assemblies 64 are shown in FIGS. 7 and 8.Each constant section fan duct 64 comprises an outer duct wall 70, aninner duct wall 72 and a plurality of separators or splitters 74 whichhold the inner and outer walls together at a constant distance againstthe great thrust exerted by the fan 52. Each of the inner and outerwalls 70, 72 and each of the splitters 74 has a sound-attenuating liner76 on the airflow surface thereof. It has been found particularlyeffective to form the splitters 74 of the acoustic liner material or tobond the material thereto.

It has been found desirable to apply a high-temperature resistantcoating 78 to the innermost surface of the inner duct wall 72. Thatsurface is spaced only a slight distance from the engine 28, whichgenerates significant heat during operation. The coating 78 preventsfire damage to the sound-attenuating liner 76. A suitable material forthis purpose is MA 25 S silicon ablative coating manufactured byMartin-Marietta Co. The outermost surface of the outer duct wall 70preferably is make of stainless steel to provide high strength within atight space.

The transition section fan duct assemblies 66 are shown in FIGS. 9 and10. Each transition section fan duct 66 comprises an outer duct wall 80,an inner duct wall 82 and a plurality of separators or splitters 84which hold the inner and outer walls together at specified distancesagainst the fan thrust. Each of the inner and outer walls 80, 82 andeach of the splitters 84 is lined or bonded with sound-attenuatingmaterial 86 on the airflow surface thereof. Similarly, it has been founddesirable to apply a high-temperature resistant coating 88 to theinnermost surface of the inner duct wall 82. The outermost surface ofthe outer duct wall 80 preferably is made of stainless steel.

The sound-attenuating liner material which has been found particularlyeffective for the purpose of the present invention is a single-layer"DynaRohr" liner material made by Rohr Industries, Inc. That material isshown and described, for example in U.S. Pat. No. 4,379,191, thedisclosure of which is incorporated herein by reference. The acousticliners may be fabricated of many materials and in different sizes andstrengths, depending upon specific operating conditions. As shown inFIG. 6, the sound-attenuating liner material preferably comprises aplurality of honeycomb core cells 90, sandwiched between a solidaluminum sheet 92 and a perforated aluminum sheet 94. A woven wire mesh96 covers the perforated sheet 94 and forms a smooth surface for laminarairflow. The honeycomb core cells 90 communicate with the atmospherethrough the perforated sheet 94 and the woven wire mesh 96. As is knwonto those skilled in the art, material of this type may be turned toparticular noise frequencies to provide effective acoustic performance.The particular preferred material also provides aerodynamic smoothnessand structural integrity. The smooth surface avoids drag, which impedesperformance and increases fuel consumption. The acoustic liner giveslinear performance and is a broad-band noise absorber. Although thematerial is sometimes referred to as a "liner", the material in fact isa structural substitute for a solid duct wall and is structurallyintegrated with the other nacelle components.

The nacelle system of the present invention has been found effective inreducing the noise levels at take-off and landing to compliance with thecurrent FAR 36. On June 28, 1985, The FAA issued a Supplemental TypeCertificate ("STC") approving use of the present invention in connectionwith Pratt & Whitney turbofan JT3D-3B engines in Douglas DC-8-62 andDC-8-62F airplanes. The following Table 1 shows the reduced noise levelsachieved with a quiet nacelles at take-off, sideline and landing in aDC-8-62 airplane with JT3D-3B engines at various maximum take-off andlanding gross weights, while Table 2 shows comparable information forunmodified nacelles:

                  TABLE 1                                                         ______________________________________                                        Max. Take-                                                                            Max Landing                                                                              Quiet Nacelles                                             off Gross                                                                             Gross      Take-Off Sideline Landing                                  Wt (lbs)                                                                              Wt (lbs)   (EPN dB)  (EPN dB)                                                                              (EPN dB)                                 ______________________________________                                        350,000 250,000    104.3     98.1    108.3                                    350,000 240,000    104.3     98.1    108.3                                    335,000 250,000    102.5     98.2    108.3                                    335,000 240,000    102.5     98.2    108.3                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Max. Take-                                                                            Max. Landing                                                                             Unmodified Nacelles                                        off Gross                                                                             Gross      Take-Off Sideline Landing                                  Wt (lbs)                                                                              Wt (lbs)   (EPN dB)  (EPN dB)                                                                              (EPN dB)                                 ______________________________________                                        350,000 250,000    111.0     103.0   114.0                                    335,000 250,000    110.0     103.0   114.0                                    ______________________________________                                    

Similarly, on July 15, 1985, the FAA issued an STC approving use of thepresent invention in connection with Pratt & Whitney turbofan JT3D-7engines in Douglas DC-8-63 and DC-8-63F airplanes. The following Table 3shows the reduced noise levels achieved with the quiet nacelles, whileTable 4 shows generally comparable information for unmodified nacellesfor that aircraft and engines:

                  TABLE 3                                                         ______________________________________                                        Max. Take-                                                                            Max. Landing                                                                             Quiet Nacelles                                             off Gross                                                                             Gross      Take-Off Sideline Landing                                  Wt (lbs)                                                                              Wt (lbs)   (EPN dB)  (EPN dB)                                                                              (EPN dB)                                 ______________________________________                                        355,000 262,000    104.1     98.2    108.5                                    355,000 258,000    104.1     98.2    108.5                                    355,000 245,000    104.1     98.2    108.4                                    ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Max. Take-                                                                            Max. Landing                                                                             Unmodified Nacelles                                        off Gross                                                                             Gross      Take-Off Sideline Landing                                  Wt (lbs)                                                                              Wt (lbs)   (EPN dB)  (EPN dB)                                                                              (EPN dB)                                 ______________________________________                                        355,000 275,000    113.9     102.8   114.3                                    ______________________________________                                    

The reduced noise levels shown in Table 1 and 3 meet both FAR 36 noiselevel requirements and ICAO Annex 16 requirements.

The effect of the reduced noise levels which can be achieved by thepresent invention is demonstrated graphically in FIG. 2. The drawingshows projected noise "footprints" or contours produced during landingand take-off both by the quiet nacelle system of the invention and by anunmodified nacelle system. The runwayy 100 forms the base line. Thenoise level contour 102 is for the unmodified system and the muchsmaller noise level counter 104 is for the system in accordance with thepresent invention. It should be appreciated that a reduction of 10 EPNdBcorresponds to a reduction of 50% in the total perceived noise level.

The components of the invention are advantageously packaged together asa "hush kit". A husk kit is a group of components which are substitutedfor existing components in a nacelle, such that the nacelle may beretrofitted without replacement of the entire nacelle system. Thus, thehush kit may comprise, within a single package, the elongated nose cowl20 with the sound-attenuating liner 42; the nose dome extension 54having the sound-attenuating liner 56; a pair of constant section fanducts 64 having the sound-attenuating liners 76; and a pair oftransition section fan ducts 66 having the sound-attenuating liners 86.The hush kit contains all the necessary major components to retrofit anexisting nacelle system. The total weight increase due to the modifiedcomponents is only about 250 lbs. per nacelle.

The invention has been described with particularity for a DC-8 airplane,as shown in FIG. 1. It has been found that the invention is especiallyeffective when used for Pratt & Whitney JT3D-3B and JT3D-7 low by-pass,fan jet engines used in nacelles for DC-8-62 and DC-8-63 airplanes.However, the invention is applicable to a variety of long duct nacellesystems used with somewhat different engines and airplanes includingDC-8-55 and DC-8-61 airplanes. The detailed description is intended tobe illustrative of quiet nacelle systems using the present invention inthe preferred manner. Nevertheless, it should be appreciated thatvarious modifications could be made in nacelle systems which remainwithin the spirit and scope of the invention. Many other uses of theinvention should be apparent to those working in the industry who areskilled in the art.

We claim:
 1. In an aircraft jet engine nacelle adapted to permitoperation of the aircraft at low noise levels at take-off and landing,the combination comprising:(a) an elongated nose cowl defining an inlet,the nose cowl having an air flow surface with an acoustic liner onsubstantially the entire airflow surface thereof and an anti-icingportion at the fowardmost portion of the nose cowl and having noacoustic liner thereon, (b) a nose dome on a fan section for the enginewith no acoustic liner thereon, and a nose dome extension disposedbetween the nose dome and the fan section, the extension having anairflow surface with an acoustic liner on the airflow surface thereof,(c) a long fan duct extending from the fan section to a thrust reverser,the fan duct comprising inner and outer wall portions and splittersdisposed between the inner and outer wall portions with the inner andouter wall portions having airflow surfaces with acoustic liners on theairflow surfaces thereof and the splitters having air flow surfaces withacoustic liners on the airflow surfaces thereof, (d) forward and aftcowls circumscribing the engine, and (e) the thrust reverser and anozzle defining an exhaust.
 2. The combination as in claim 1 wherein thefan duct comprises constant section ducts.
 3. The combination as inclaim 1 wherein the fan duct comprises transition section ducts.
 4. Thecombination as in claim 1 wherein the acoustic liner is a single layerhoneycomb sandwich acoustic material.
 5. The combination as in claim 1wherein the acoustic liner comprises a honeycomb core cell structurehaving a perforated aluminum sheet covered by a woven wire mesh on theairflow side thereof and a solid aluminum sheet on the rear sidethereof.
 6. The combination as in claim 1 wherein the inner wallportions of the fan duct have a high-temperature resistant coating onthe side adjacent the engine.
 7. A quiet nacelle system for a jetairplane, comprising:(a) a long fan duct jet engine having an inlet andexhaust and fan, combustion and turbine sections, (b) an inlet cowlcircumscribing the inlet, the inlet cowl comprising an outer barrel, aninner barrel formed of sound-attenuating material and positioned withinthe outer barrel, an anti-icing lip assembly at the forwardmost portionof and connecting the inner and outer barrles and an aft bulkhead at therearwardmost portion of the inner and outer barrels, (c) a nose domewithin the inlet cowl and having an extension at the rearwardmostportion thereof, the extension being formed of sound-attenuatingmaterial, (d) forward and aft cowls trailing the inlet cowl andcircumscribing the fan, combustion and turbine sections of the engine,and (e) a long fan duct assembly positioned within the forward and aftcowls and comprising bifurcation, constant section, transition sectionand aft section ducts, the constant section and transition section ductsbeing comprised of inner and outer duct walls connected by a pluralityof separating means, the inner and outer duct walls and separating meansbeing formed of sound-attenuating material and the inner walls beingcoated with temperature-resistant means,whereby the elements of thesystem act synergistically to reduce the engine noise to a quiet levelat take-off and landing while maintaining airworthiness, structuralintegrity and safe flight performance of the airplane.
 8. A hush kit foran aircraft jet engine nacelle, comprising:(a) an elongated nose cowladapted to circumscribe an inlet of the engine and having an airflowsurface comprised of sound-attenuating material; (b) an extended nosedome portion adapted to be attached to a fan of the engine and having anairflow surface comprised of sound-attenuating material; (c) a pair ofconstant-section fan ducts adapted to be disposed between the fan andtransition-section fan ducts for the engine and having airflow surfacesand splitters arranged within the fan ducts, the surfaces and splittersboth being comprised of sound-attenuating material; and (d) a pair oftransition-section fan ducts adapted to be disposed between theconstant-section fan ducts and rear of the engine and having airflowsurfaces and splitters arranged within the fan ducts, the surfaces andsplitters both being comprised of sound-attenuating material,whereby thecomponents of the kit are adapted to be substituted for existingcomponents in the nacelle to facilitate reduction of engine noise duringtake-off and landing of the aircraft.